The present invention relates to a sensing device of a strain gauge type for measuring a load on, for example, a truck, a method of correcting an output from the sensing device, and an apparatus therefor.
In recent years, there arises a problem that a vehicle or the like having a load superimposed thereon has a traffic accident, such as rolling over, and the vehicle and the road are excessively damaged.
As one of the causes of the above-mentioned problem, an overload has been considered. To prevent the overload, the load on a vehicle, that is, a load superimposed on the vehicle has been measured.
Hitherto, a load on a vehicle has been measured such that a vehicle, the load of which is required to be measured, is placed on a platform scale. However, a large scale facility must be provided, which requires a large area, thus resulting in a limited number of the platform scales permitted to be provided. Therefore, a large number of vehicles cannot be measured and the cost of installation cannot be reduced.
Accordingly, a load measuring apparatus mounted on a vehicle and capable of measuring the load has been provided in recent years.
For example, in Japanese Patent Application No. Hei. 7-124860, as shown in the FIG. 9 perspective view, attention is paid to the fact that a shackle 134 is disposed between an end of a leaf spring 131 connected to a frame 132 of a bed through a forked bracket 133 and another bracket 133 connected to the frame 132. Then, a sensing device of a strain gauge type, for example, a strain gauge sensor for measuring a load is disposed in a shackle pin 5 for movably connecting the shackle 134 to the bracket 133. The load is calculated in accordance with the total amount of measured values of a plurality of sensing devices corresponding to respective wheels.
FIG. 10 is a cross sectional view showing positions of sensing devices in the shackle pin. The shackle pin 5 has, at the two ends in the axial direction thereof, accommodating spaces 5a and 5b. The accommodating spaces 5a and 5b communicate with each other through a through hole 5c in the shackle pin 5.
The accommodating space 5a of the shackle pin 5 accommodates a sensing device 7A accommodated in a first case assembly 9 in such a manner that the first case assembly 9 is accommodated. The accommodating space 5b of the shackle pin 5 accommodates a sensing device 7A accommodated in a second case assembly 11 in such a manner that the second case assembly 11 is accommodated.
The conventional sensing devices 7A respectively accommodated in the two accommodating spaces 5a and 5b of the shackle pin 5, shown in FIG. 11 as a perspective view, comprise a plate-like member 7a and a coil 7g serving as a sensing section.
The plate-like member 7a is made of a magnetic material, such as permalloy, formed into a substantially rectangular shape and comprises a coil portion 7b a substantially central portion in the lengthwise direction and fixed portions 7c on both sides of the coil portion 7b.
Four cut portions 7e each having a semicircular shape for relaxing stress acting on the plate-like member 7a are formed at the boundaries between the coil portion 7b and the two fixed portions 7c in the two end surfaces in the widthwise direction of the plate-like member 7a. The coil portion 7b inside the four cut portions 7e has four through holes 7f respectively formed apart from one another in the lengthwise direction and the widthwise direction.
The coil 7g has an exciting winding 7h and a detecting winding 7j forming a cross coil, and the windings 7h and 7j are covered with an insulating material (not shown), such as enamel.
The detecting winding 7j is wound between the two through holes 7f opposite to each other in the lengthwise direction. The two ends of the detecting winding 7j are extracted through the through holes 7f. The exciting winding 7h is wound between the two through holes 7f opposite to each other in the widthwise direction. The two ends of the exciting winding 7h are extracted through the through holes 7f.
The sensing devices 7A having the above-mentioned structure are, in a state where they are accommodated in the first case assembly 9 and the second case assembly 11, usually disposed in the accommodating spaces 5a and 5b of the shackle pin 5 in such a way that, as shown in FIG. 10, the lengthwise direction of the plate-like member 7a coincides with the axial direction of the shackle pin 5 which extends in the widthwise direction B of the vehicle and the widthwise direction of the plate-like member 7a coincides with the direction A of the height of the vehicle. Moreover, one of the fixed portions 7c is received by the bracket 133, while the other of the fixed portions 7c is received by the shackle 134.
In the above-mentioned state, both of the direction of the axis winding of the detecting winding 7j wound between the two through holes 7f in the lengthwise direction of the plate-like member 7a and the direction of the axis of winding of the exciting winding 7h wound between the two through holes 7f opposite to each other in the widthwise direction of the plate-like member 7a are made to be a direction of a plane including the lengthwise and widthwise directions of the plate-like member 7a, that is, a direction of a plane along the direction A of the height and direction B of the width of the vehicle.
When an electric current is allowed to flow between the two ends of the exciting winding 7h of the sensing device 7A, a magnetic field is generated in the plate-like member 7a so that an induced current flows between the two ends of the detecting winding 7j.
When a load is applied from the frame 132 to the plate-like member 7a through the bracket 133, the shackle 134, and the two ends of the shackle pin 5, so that, the bracket 133 is relatively moved in the direction A of the height of the vehicle with respect to the shackle 134 and the plate-like member 7a is also distorted in the direction A, the direction of the magnetic field in the sensing device 7A is changed. As a result, the induced current, which flows between the two ends of the detecting winding 7j, is changed.
As a result, an electric current, the level of which corresponds to the load applied to the sensing device 7A, can be obtained from the two ends of the detecting winding 7j as an output signal.
However, if the ambient temperature is changed or the sensing device 7A deteriorates with age, the output characteristic of the above-mentioned conventional sensing device 7A is changed. As a result, the level of the induced current, which flows between the two ends of the detecting winding 7j when a load is applied, is undesirably changed even if the level of the electric current, which is supplied to the exciting winding 7h, is kept constant. Therefore, there arises a problem that an accurate load cannot be calculated in accordance with the output from the sensing device 7A.
It might therefore be considered to employ a temperature compensating method having the steps of detecting the ambient temperature by using, for example, a temperature sensor, and correcting the level of the induced current, which flows between the two ends of the detecting winding 7j, to correspond to the change in the temperature. However, change in the characteristic of the sensing device 7A attributable to deterioration with age cannot be compensated.